1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device configured to prevent a track error signal offset to allow stable tracking.
2. Description of the Related Art
In general, an optical pickup device is configured as shown in FIG. 1 to detect a track error signal such that a beam emitted from a light source 10 tracks the proper position of a recording medium such as a disk 1 to record and/or reproduce an information signal, and is configured to control an objective lens 13 in the radial direction of the disk 1 according to the detected signal so that a light spot lands on the proper track position.
A track error signal is detected as a push-pull signal, for example, which is obtained by independently receiving light radiated from the light source 10 and reflected from the disk 1 at two light receiving regions A and B of a photodetector 15, converting the incident light into electrical signals, applying the detected signals of the light receiving regions A and B to input ports of a differential unit 17 and obtaining a difference therebetween.
Here, the light spot formed on the disk 1 by an objective lens 13 causes a O-order diffracted beam and xc2x11st-order beams to interfere according to the location from the track center where it is reflected from the disk 1. Thus, the thus-obtained push-pull signal is proportional to sin(2xcfx80/tp) where tp designates the track pitch.
Here, reference numerals 11, 12 and 14 represent a collimating lens, a beam splitter and a light receiving lens, respectively.
The above-described conventional optical pickup device forms a light spot at the proper track position of the disk 1 by moving the objective lens 13. Thus, the light which is reflected from the disk 1 to then be incident on a photodetector 15 is also shifted as the objective lens 13 is moved, resulting in an offset to a track error signal.
To solve the above problems, it is an object of the present invention to provide an optical pickup device configured to be capable of tracking in a stable manner without moving an objective lens laterally, so as not to generate an offset in a track error signal.
To achieve the above object, there is provided an optical pickup device including a light source for generating light, a light path changing means for changing the traveling path of incident light, an objective lens for focusing incident light to form a light spot on a recording medium and having an incidence region of a predetermined size, a tracking mirror disposed on the light path between the light path changing means and the objective lens, for reflecting incident light emitted from the light source so that the light spot traces proper track positions of the recording medium, a lens unit consisting of at least two lenses which allow the light reflected from the tracking mirror to be incident on the incidence region of the objective lens, and a photo detecting unit for receiving light reflected from the recording medium and having passed through the objective lens and the light path changing means.
Here, the lens unit may include a collimating lens disposed on the light path between the light source and the objective lens, for focusing incident light emitted from the light source, and a relay lens disposed on the light path between the collimating lens and the objective lens, for convergently refracting light reflected by the tracking mirror to be incident on the incidence region of the objective lens.
In this case, the collimating lens and the relay lens are preferably provided such that incident light is focused to a point and then allowed to diverge to be incident on the relay lens.
The objective lens may include a first transmission portion forming the incidence region, for spreading incident light, a first reflection portion spaced a predetermined distance apart from the first transmission portion, for reflecting divergent incident light incident from the first transmission portion, a second reflection portion provided around the first transmission portion, for further reflecting the light reflected from the first reflection portion and focusing the same, and a second transmission portion provided at an interior or exterior side of the first reflection portion, for transmitting incident light to be focused on the recording medium.
According to one aspect of the present invention, the optical pickup device may further include a polarization prism for separating according to polarization incident light reflected from the recording medium and having passed through the objective lens and the light path changing means, the optical pickup device configured for recording on and/or reproducing from a magneto-optical recording medium such that the light path changing means comprises a partial polarized beam splitter for transmitting and reflecting a polarized light component in a predetermined ratio among incident light components and mostly transmitting or reflecting the other polarized light component, and such that the photo detecting unit comprises first and second photo detectors for receiving different polarized light components whose paths are changed by the partial polarized beam splitter and which are separated by the polarization prism.
According to another aspect of the present invention, the light path changing means may include a polarization beam splitter for transmitting and reflecting incident light according to its polarization, and a wave plate disposed on the light path between the polarization beam splitter and the objective lens, for changing the polarization of the incident light.